The minimum system requirements for Windows 7 state that 16GB of hard drive space is needed for 32-bit versions and 20GB for 64-bit versions. Since one generally runs Internet Explorer in 32-bit even on 64-bit systems, you probably want to go with 32-bit, and thus have at least 20GB so you have a little breathing room for the cache, any downloading you might do, plugins or extensions you might install, to update Internet Explorer to version 9, and any operating system updates that will be necessary.

That being said, VMware generally uses sparse disk images (unless you specifically disable them), which means regardless of how large you make the virtual disk image, it will only take up the space on your hard drive that you're actually using. So there's no reason not to be generous.

OK, I'll definitely go with 32-bit. Are there features of Windows 7 I can disable that will make the OS smaller on disk? (e.g. unchecking boxes in the installer)
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Dan FabulichMar 13 '11 at 1:48

@Dan: Unlike previous versions of Windows, Windows Vista and 7 do not allow you to select components to install during Windows Setup. There is an option in the Control Panel to disable and enable access to certain components, but it does not remove them from disk. Removing access may reduce disk usage slightly because Windows will not need to store shortcuts and settings for them, and it may compress the files needed to run them (but I'm not sure about that).
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PatchesMar 13 '11 at 2:00